1. Field of the Invention
The present invention relates to an optical communication device used in wavelength division multiplexing (WDM) optical communication networks, and more particularly to a wavelength division multiplexer/demultiplexer for multiplexing/demultiplexing optical signals.
2. Description of the Related Art
Generally, WDM optical communication networks are used to transmit large amounts of data, the optical signals, which include multiple channels, are simultaneously transmitted using a single strand of optical fiber. A transmitting terminal within the WDM network serves to multiplex an optical signal. A receiving terminal within the WDM network serves to receive the multiplexed optical signal and separate it into optical signals of different wavelengths as well as to change the received optical signal into respective electrical signals. In a WDM optical communication network based on a single-mode optical fiber, an Arrayed Waveguide Grating (AWG) (using a planar lightwave circuit (PLC) technique) is used in the wavelength division multiplexer/demultiplexer. The planer lightwave circuit comprises a substrate made of silicon or quartz, and a plurality of silica layers or polymer thin films deposited on the substrate.
In demultiplexing the optical signal, the shape of the input waveguide of the multiplexed optical signal determines the bandwidth and quality of the optical signal.
FIG. 1 is a schematic of an input waveguide of a conventional wavelength division multiplexer/demultiplexer. As shown in FIG. 1, the input waveguide 313 of the conventional wavelength division multiplexer/demultiplexer inputs a multiplexed optical signal to a grating of the wavelength division multiplexer/demultiplexer. However, when the wavelength division multiplexer/demultiplexer is operated as a demultiplexer, the input waveguide 313 outputs the multiplexed optical signal.
The input waveguide 313 has a parabolic horn shape, in which its width (W1) is gradually increased in a progressing direction of the optical signal. Variation of width (W1) of the input waveguide 313 is defined below by Equation 1, and the shape of input waveguide 313 is defined below by Equation 2.
                                          1            2                    ⁢                                    ⅆ                              W                1                                                    ⅆ              z                                      =                              -            γ                    ⁢                                          ⁢                                    4              ⁢                                                          ⁢              π                        3                    ⁢                                    λ              g                                      W              il                                                          [                  Equation          ⁢                                          ⁢          1                ]                                                      W            1                    =                                    (                                                2                  ⁢                                                                          ⁢                  α                  ⁢                                                                          ⁢                                      λ                    g                                    ⁢                                      z                    1                                                  +                                  W                                      i                    ⁢                                                                                  ⁢                    1                                    2                                            )                                      1              /              2                                      ,                  α          =                      -                                          8                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                γ                            3                                                          [                  Equation          ⁢                                          ⁢          2                ]            
Herein, z1 denotes the distance from an input terminal of input waveguide 31 to a position of the progressing optical signal. W1 is the width of input waveguide 313 at the position of the progressing optical signal. Wi1 is the width of input waveguide 313 at the input terminal. λg denotes the effective wavelength of the optical signal and γ denotes the coupling coefficient of the fundamental and higher modes of the optical signal.
When the multiplexed optical signal progresses along the parabolic horn type input waveguide 313, mode coupling of the optical signal from the fundamental mode to the secondary mode or the higher mode occurs. In addition, the bandwidth of the optical signal is expanded. Consequently, the optical signal has a flat field profile. Equation 3 below illustrates a relationship between the coupling coefficient created by the optical signal progressing along input waveguide 313 and the maximum value of power (Pj) of the optical signal coupled from the fundamental mode to the higher mode. FIG. 5 is a graph illustrating the power (Pj) profile of the coupled optical signal from the fundamental mode to the higher mode as the multiplexed optical signal progresses along input waveguide 313.
                                          P            j                                P            0                          =                                            4              ⁢                                                          ⁢                              γ                2                                                                    4                ⁢                                                                  ⁢                                  γ                  2                                            +              1                                =                                                    (                                  3                  ⁢                                                                          ⁢                                      α                    /                    4                                    ⁢                                                                          ⁢                  π                                )                            2                                                                        (                                      3                    ⁢                                                                                  ⁢                                          α                      /                      4                                        ⁢                                                                                  ⁢                    π                                    )                                2                            +              1                                                          [                  Equation          ⁢                                          ⁢          3                ]            
Herein, Pj denotes the power of an optical signal coupled from the fundamental mode to the higher mode. P0 denotes the power of an optical signal initially inputted to input waveguide 313. As shown in Equation 3 and FIG. 5, the power (Pj) of the optical signal coupled from the fundamental mode to the higher mode varies according to the width (Wi1) of input waveguide 313 at the input terminal and the width (W1) of input waveguide 313 at a certain position.
Such a conventional parabolic horn type input waveguide 313 has several limitations. In particular, as the bandwidth of the optical signal expands, interference of the optical signal occurs from an optical signal of a neighboring channel. Thus, the quality of the optical signal is depreciated. Further, since mode coupling of the optical signal from the fundamental mode to the higher mode occurs, a side lobe is generated. The interference between the neighboring channels generated by the side lobe is a leading factor depreciating the quality of the optical signal.